The present invention relates to a method for monitoring wheel speeds, having the following steps: Determining the wheel speeds, determining the wheel having the maximum wheel speed, determining the wheel having the minimum wheel speed, determining the speed difference between the maximum wheel speed and minimum wheel speed, and comparing the speed difference to a threshold value. The present invention further relates to an apparatus for monitoring wheel speeds, having means for determining the wheel speeds, means for determining the wheel having the maximum wheel speed, means for determining the wheel having the minimum wheel speed, means for determining the speed difference between the maximum wheel speed and the minimum wheel speed, and means for comparing the speed difference to a threshold value.
In vehicle safety systems, for example in the electronic stability program (ESP), the rotation speed sensor signals are elementary input variables. From the rotation speed sensor signals and with the aid of additional sensor signals, the free-rolling wheel speeds referred to the center of the rear axle are calculated. Using the additional sensor signals it is possible to refer the wheel speeds to the vehicle center of gravity or to the center of the rear axle. These wheel speeds referred to the vehicle center of gravity or the center of the rear axle need to be exactly monitored for numerous functions, for example for the ABS (antilock braking system) function or ASR (automatic slip control) function. It is necessary to monitor the wheel speeds very accurately in particular for the ASR portion of the ESP, since narrow activation thresholds are used for these functions.
In any event, it is advantageous to detect erroneous signals that may occur, for example, because of incorrect or defective gears or also as a result of failed rotation speed sensors or an excessive gap dimension.
According to the existing art, monitoring of the wheel slip is accomplished on the basis of a logic system which is active above the vehicle reference speed of 18 km/h. At 20 km/h, the difference between the slowest and fastest wheel speeds must be no greater than 2 km/h. This value corresponds to 10%. At 100 km/h, the difference must be no greater than 5 km/h. This value corresponds to 5%. Between 18 km/h and 100 km/h, the permissible differences are correspondingly linearly extrapolated or interpolated. Above 100 km/h, the slip difference between one wheel and the other three wheels must not be greater than 5% in each case. If the difference is greater than indicated by the above limit values, a fault counter is incremented. After a specific time period, for example 20 seconds, a fault is identified as present.
In the existing logic system, it is problematic that the monitoring is performed solely on the basis of the slowest and fastest wheel speeds. At vehicle speeds below 100 km/h in particular, robustness problems can therefore occur on irregular roads or, for example, in deep snow. The existing logic system does not contain a so-called deep snow detector.
The present invention expands upon the method of the species in that deep snow or road conditions of comparable effect are inferred if the speed difference is greater than a threshold value and/or the slowest wheel has changed exactly once and the fastest wheel has changed exactly once or the slowest wheel has not changed and the fastest wheel has not changed and the speed difference between a driven axle and a non-driven axle exceeds a threshold value. The axle speeds can be calculated, for example, as the average of the wheel speeds of the wheels arranged on an axle. The threshold value can be selected, for example, in such a way that if the driven axle exhibits a speed that is 4 km/h greater than that of the non-driven axle, deep snow is identified as present.
Preferably, deep snow or road conditions of comparable effect on one side are inferred if the difference between the averaged wheel speeds of the vehicle sides exceeds a threshold value. This threshold value can be set, for example, at 2 km/h.
It is also advantageous if deep snow or road conditions of comparable effect on both sides are inferred if a tire tolerance compensation has been performed. It is thus possible to identify as present both deep snow on one side and deep snow on both sides, and consequently to decrement a fault counter of the slip monitoring system for the rotation speed sensor signals. Greater robustness of the control system is thereby obtained, this advantageously being seen in particular in the context of deep snow, aquaplaning, mud, sand, or similar road conditions.
The present invention is advantageous in particular because deep snow or road conditions of comparable effect are inferred on the basis of a wheel logic in which, if the speed difference is greater than a threshold value, the wheel having the maximum wheel speed is recorded as the fastest wheel and the wheel having the minimum wheel speed is recorded as the slowest wheel; in the event of a change in wheel speed, so that a different wheel is the fastest wheel and/or a different wheel is the slowest wheel, the fastest wheel is recorded and the slowest wheel is recorded; and from the recorded values and the present values, a logical status is ascertained. This wheel logic increases the robustness of the control system, since the monitoring is not taking place solely on the criterion of the slowest and fastest wheel speeds. Instead, the system records whether a change in the slowest or fastest wheel takes place. On the basis of the various cases that can occur, a corresponding status can be ascertained for the logical combinations that occur. On the basis of such a status, a probable number of faulty wheels can be inferred. In addition, a fault counter can be incremented or decremented depending on whether a fault, and which fault, is present. In particular, the fault counter can be decremented if deep snow on one side or both sides was identified as present.
Preferably, status 0 is identified as present if the slowest wheel and/or the fastest wheel have changed more than once. If the slowest wheel and/or fastest wheel changes more than once during monitoring, an implausibility is then present, such as can occur on a particular stretch of road with an uneven (xe2x80x9cwashboardxe2x80x9d) surface. In a situation that corresponds to status 0, however, it is not possible to infer an actual fault.
Advantageously, status 1 is identified as present if only the slowest wheel has not changed or if only the fastest wheel has not changed. If the slowest or the fastest wheel was therefore hitherto always the same wheel, this indicates a single faulty wheel.
It is preferred if status 2 is identified as present if the slowest wheel has changed exactly once and the fastest wheel has changed exactly once. In status 2, two faulty wheels are therefore probably present. The test for a difference in axle speeds is performed when status 2 exists, however, and a check is also made as to whether different average speeds are present on the two sides of the vehicle or whether a tire tolerance compensation has been performed. If a specific minimum difference exists between the axle speeds, as well as a difference between the speeds of the wheels on the two sides of the vehicle, deep snow on one side is inferred. If a tire tolerance compensation was performed, deep snow on both sides is inferred.
It is further preferred if status 3 is identified as present if the slowest wheel has not changed and if the fastest wheel has not changed. In such a situation, one or two faulty wheels are inferred. In this case as well, a check is made for the presence of deep snow. Deep snow on one side is present if, in addition to the identification of status 3 as present, the difference between the axle speeds exceeds a threshold value, and the difference between the averaged wheel speeds on the sides of the vehicle exceeds a threshold that is also predetermined.
It is advantageous if when status 0 exists, a fault counter is decremented. Since a fault cannot be inferred in status 0xe2x80x94it would be happenstance if a fault were present simultaneously with the identification of status 0 as presentxe2x80x94a fault counter is decremented. If the fault counter is reset to 0 as a result of this decrementing of the fault counter, the recorded wheels are then deleted. The wheel logic is thus reinitialized.
Preferably, when status 1 and status 3 exist a fault counter is incremented by 2. After a specific time period, for example 20 seconds, a fault can thus be identified as present. The fault counter is decremented, however, if deep snow was identified as present while status 3 was present.
It is preferred to increment a fault counter by 1 when status 2 exists. In status 2, a fault can therefore be identified as present after, for example, 40 seconds. The fault counter is decremented, however, if deep snow was identified as present when status 2 existed.
It is useful that the wheel speeds are determined from rotation speed sensor signals and additional sensor signals. With the combination of these signals, it is possible to determine wheel speeds referred to the vehicle center of gravity or to the center of the rear axle, which are particularly well suited for slip monitoring.
The method according to the present invention is advantageous in particular because of the fact that the speed difference is compared to a relative threshold. For example, the relative threshold can be 5% over the entire speed range between 20 km/h and 100 km/h.
Another possibility, however, is that below a speed threshold, the speed difference is compared to an absolute threshold value. The speed threshold below which a constant value is used can lie, for example, at 40 km/h, a speed difference threshold of, for example, 2 km/h between the slowest and fastest wheel being the criterion for initiating monitoring with the wheel logic according to the present invention.
The present invention expands upon the apparatus of the species in that deep snow or road conditions of comparable effect are inferred if the speed difference is greater than a threshold value and the slowest wheel has changed exactly once and/or the fastest wheel has changed exactly once or the slowest wheel has not changed and the fastest wheel has not changed and the speed difference between a driven axle and a non-driven axle exceeds a threshold value. The axle speeds can be calculated, for example, as the average of the speeds of the wheels arranged on an axle. The threshold value can be selected, for example, in such a way that if the driven axle has a speed that is 4 km/h greater than that of the non-driven axle, deep snow is identified as present.
The present invention is based on the recognition that a monitoring function with improved robustness can be made available by introducing a wheel logic system. This applies in particular to monitoring of the slip control system on unusual (xe2x80x9cwashboardxe2x80x9d) road surfaces, or in the context of other special road conditions such as deep snow.